Technical Field
The present invention relates to a power converter, which drives a switch on the basis of a current command and a current that flows therein.
Related Art
A DC (direct current) to DC converter (hereinafter referred to as “DC-DC converter”) may be used as a power converter that drives a switch on the basis of a current command and a current that flows, as disclosed in, e.g., JP-A-2009-118571.
This DC-DC converter is adapted to control an output of a driving signal on the basis of a target current and a current flowing in a transformer so as to drive a transistor. The DC-DC converter includes a central processing unit (CPU), a digital to analog (D/A) converter, a current sensor, an absolute value circuit, a comparator, a holding circuit, an AND circuit, and a driving circuit. The CPU generates a target current on the basis of a difference voltage between a target voltage and an output voltage. The D/A converter converts the generated target current to a voltage. The current sensor and the absolute value circuit detect an absolute value of a current flowing in a transformer and output the detected absolute as a voltage. The comparator compares the absolute value of the current flowing in the transformer with the target current. The CPU includes a pulse outputting section that outputs pulse signals with a predetermined duty cycle in a predetermined cycle. The holding circuit and the AND circuit control an output of a driving signal for a transistor on the basis of a comparison result of the comparator. If the absolute value of the current flowing in the transformer is smaller than the target current, the holding circuit and the AND circuit output the pulse signal of the pulse outputting section as the driving signal for the transistor. If the absolute value of the current flowing in the transformer is larger than the target current, the holding circuit and the AND circuit stop the output of the driving signal to the transistor. The driving circuit turns the transistor on and off on the basis of the driving signal outputted from the holding circuit and the AND circuit.
In addition, there is a DC-DC converter that controls the timing for turning a transistor off on the basis of a target current and a current flowing in a transformer. This DC-DC converter includes a reference signal generator, a current detector, a reset signal generator, and a driving unit. The reference signal generator generates a reference signal at every predetermined cycle and outputs the generated the reference signal. The current detector detects an absolute value of a current flowing in a transformer and outputs the detected absolute value. The reset signal generator generates a reset signal on the basis of a result of a comparison between a target current and the absolute value of the current flowing in the transformer and outputs the generated reset signal. If the absolute value of the current flowing in the transformer is larger than the target current, the reset signal generator outputs the reset signal. The driving unit turns a transistor on in synchronization with the reference signal. If the reset signal is outputted before the subsequent reference signal is outputted, the driving unit turns the transistor off in synchronization with the reset signal. If the reset signal is not outputted before the subsequent reference signal is outputted, the driving unit turns the transistor off in synchronization with the subsequent reference signal.
In the current detector, there is a delay between when the input current is detected and when it is outputted. In the reset signal generator also, there is a delay until the reset signal is outputted. Due to these delays, an output timing of the reset signal is delayed. Thus, the reset signal, which is needed to be outputted immediately before the reference signal is outputted, becomes outputted immediately after the reference signal is outputted. As a result, the transistor, which is turned on in synchronization with the reference signal, becomes turned off in synchronization with the reset signal which is outputted immediately after the reference signal is outputted due to these delays. That is, the transistor performs a switching operation different from the desired switching operation. In this case, a current that originally flows does not flow, and then, an input current and an output voltage are greatly fluctuated.
In addition, in the driving unit, there is a delay between when the reset signal is detected and when the transistor is turned off. In the transistor also, there is a delay until it is turned off. Due to this, even when the reset signal is outputted immediately before the reference signal is outputted, the transistor that is turned on in synchronization with the reference signal may become turned off immediately after the reference signal is outputted due to these delays. That is, the transistor performs a switching operation different from the desired switching operation. In this case also, a current that originally flows does not flow, and then, an input current and an output voltage are greatly fluctuated.